Narrowband filters are particularly useful in the communications industry and particularly for wireless communications systems which utilize microwave signals. At times, wireless communications have two or more service providers operating on separate bands within the same geographical area. In such instances, it is essential that the signals from one provider do not interfere with the signals of the other provider(s). At the same time, the signal throughput within the allocated frequency range should have a very small loss.
Within a single provider's allocated frequency, it is desirable for the communication system 20 to be able to handle multiple signals. Several such systems are available, including frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and broad-band CDMA (b-CDMA). Providers using the first two methods of multiple access need filters to divide their allocated frequencies in the multiple bands. Alternatively, CDMA operators might also gain an advantage from dividing the frequency range into bands. In such cases, the narrower the bandwidth of the filter, the closer together one may place the channels. Thus, efforts have been previously made to construct very narrow bandpass filters, preferably with a fractional-band width of less than 0.05%.
An additional consideration for electrical signal filters is overall size. For example, with the development of wireless communication technology, the cell size (e.g., the area within which a single base station operates) will get much smaller—perhaps covering only a block or even a building. As a result, base station providers will need to buy or lease space for the stations. Since each station requires many separate filters, the size of the filter becomes increasingly important in such an environment. It is, therefore, desirable to minimize filter size while realizing a filter with very narrow fractional-bandwidth and high quality factor Q.
Microstrip filters have the advantages of small size and low manufacturing costs. However, microstrip filters constructed of conventional metals suffer a much higher loss than other technologies (e.g., such as waveguide, dielectric resonator, combline, etc.), and especially in very narrow bandwidth filters. With high-temperature superconductive (“HTS”) thin film technology, microstrip filters using HTS materials can achieve extremely low loss and superior performance. Therefore, use of HTS microstrip filters is particularly useful for very-narrow band filters.
Using microstrip technology for narrow bandpass filter design, the spacing between the resonators usually determines the amount of coupling between the resonators. As the spacing increases, the coupling decreases and, therefore, the bandwidth becomes narrower. For very-narrow band filters, the spacing between resonators can be quite substantial. Techniques have been developed in the prior art to reduce the required spacing. For example, in a lumped element type resonator environment (see Zhang, et al. U.S. patent application Ser. No. 08/706,974, which issued on Aug. 20, 2002 as U.S. Pat. No. 6,438,394, and Ye, U.S. patent application Ser. No. 09/699,783, which issued on Oct. 14, 2008 as U.S. Pat. No. 7,437,187); and in a distributed element type resonator environment (see Tsuzuki, et. al., U.S. Provisional Application 60/298,339), all assigned to the assignee of the current invention. These techniques have been shown to be successful in effectively reducing the spacing between resonators for very-narrow band filters in the respective environments. However, the techniques may not be effective (using the same structure), when the required bandwidth of the filter becomes large. Where a broader bandwidth is desired, closer spacing between resonators is required. In some cases, the spacing may become too small from manufacturability point of view, i.e., lithography, sensitivity, yield, etc.
It is also known that to reach higher filter rejection performance while maintaining a minimal number of resonators, couplings between non-adjacent resonators can be applied to realize transmission zeros. For example, see MICROSTRIP CROSS-COUPLING CONTROL APPARATUS AND METHOD, filed Apr. 2, 1999, and receiving Ser. No. 09/285,350, which issued on Mar. 4, 2003 as U.S. Pat. No. 6,529,350, which application is commonly assigned to the assignee of the present application. Such application being incorporated herein and made a part hereof by reference. These transmission zeros can be placed at strategic locations to achieve optimal filter performance. Besides actual cross coupling value, the precise transmission zero location depends an the phase of these crass couplings, i.e., whether it is positive cross coupling or negative cross coupling. Therefore, crass coupling can be utilized to improve filter performance.
Therefore, there exists a need for a very-narrow bandwidth filter having the convenient fabrication advantage of microstrip filters while achieving, in a small filter, the appropriate coupling. Further, the appropriate coupling should take advantage of cross-coupling between non-adjacent resonators to introduce transmission zeros which provide an optimized transmission response of the filter.